Evolution And History

Science, Illness, and Disease 309

at certain social and economic levels. But to people liv-

ing where schistosomiasis is endemic (the public health

term for a disease that is widespread in the population),

this disease state is regarded as normal, and no treat-

ment is sought. In other words schistosomiasis is not an

illness. Individuals may know about expensive effective

biomedical treatments, but given the likelihood of re-

infection and the inaccessibility of the drugs, treatment

with pharmaceutical agents is not the social norm. Over

time, the forces of evolution generally lead to a tolerance

between parasite and host so that infected individuals

can live normal lives. So accustomed are some societies

to this parasitic infection that the appearance of bloody

Disease can also exist without illness. Schistosomia-

sis, infection with a kind of parasitic flatworm called a

blood fluke, is an excellent example. Scientists have fully

documented the life cycle of this parasite that alternates

between water snail and human hosts. The adult worms

live for many years inside human intestine or urinary

tract. Human waste then spreads the mobile phase of the

parasite to freshwater snails. Inside the snails, the parasite

develops further to a second mobile phase of the flatworm

life cycle, releasing thousands of tiny creatures into fresh-

water. If humans swim, wade, or do household chores such

as laundry in this infested water, the parasite can bore its

way through the skin, traveling to the intestine or bladder

where the life cycle continues.

The idea of parasites boring through the skin and

living permanently inside the bladder or intestine may

well be revolting; ingesting poisons to rid the body of

these parasites is an acceptable alternative for people

Building the Aswan Dam was a vital part of Egypt’s modernization. Unfortunately, the dam has also caused

problems. It increased the rates of schistosomiasis in the Nile River by creating a massive artificial lake up-

stream from the dam, thus providing the ideal environment for water snails. Downstream, the reduced flow of

Nile waters caused a loss of silt and fertile soil previously brought into the Mediterranean Sea. The silt and

soil had built up the Egyptian coastline. Thus, an unintended consequence of the dam has been soil erosion

along the coast, allowing the sea to encroach upon people’s homes.

© Reza; Webistan/Corbis

endemic The public health term for a disease that is wide-

spread in a population.

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